Digital phase locked loop circuitry and methods

ABSTRACT

Phase locked loop circuitry operates digitally, to at least a large extent, to select from a plurality of phase-distributed candidate clock signals the signal that is closest in phase to transitions in another signal such as a clock data recovery (“CDR”) signal. The circuitry is constructed and operated to avoid glitches in the output clock signal that might otherwise result from changes in selection of the candidate clock signal. Frequency division of the candidate clock signals may be used to help the circuitry support serial communication at bit rates below frequencies that an analog portion of the phase locked loop circuitry can economically provide. Over-transmission or over-sampling may be used on the transmit side for similar reasons.

This is a continuation of U.S. patent application Ser. No. 10/923,129,filed Aug. 20, 2004, which is a continuation-in-part of U.S. patentapplication Ser. No. 10/349,541, filed Jan. 21, 2003, now U.S. Pat. No.7,138,837, both of which are hereby incorporated by reference herein intheir entireties.

BACKGROUND OF THE INVENTION

This invention relates to phase locked loop (“PLL”) circuitry, and moreparticularly to digital phase locked loop (“DPLL”) circuitry.

PLL circuitry is a frequently needed type of circuitry. For example, inthe reception of clock data recovery (“CDR”) signals, PLL circuitry maybe used to help match the frequency and phase of a controllably variableclock signal to the clock information that is embedded in the receivedCDR signal. The frequency-and-phase-matched clock signal can be used asa “recovered” clock signal, which is useful, for example, in processingthe data information that is also recovered from the CDR signal.

PLL circuitry may include a “digital” portion (“DPLL circuitry”). Forexample, after a frequency match has been achieved, several versions(“candidate clock signals”) of the frequency-matched clock signal may beproduced. Each of these versions is shifted somewhat in phase relativeto the other versions. The digital portion of the PLL circuitry may beused to make a final selection of the version that has the best phasematch. Relative stability in such a final selection is important (e.g.,to avoid final selections that change too soon (prematurely) or toooften (“hunting”)). Also, it can be important to avoid “glitches” in therecovered clock signal. Glitches can be associated with certain types ofchanges in the final selection of the clock signal version to be outputas the recovered clock signal. A glitch is typically one or more signaltransitions that are fragmentary or too close to one another or to othertransitions in the recovered clock signal (i.e., signal transitionspacings that are too small a fraction of a proper recovered clocksignal cycle).

Although there is a great deal of interest in CDR circuitry that cansupport serial communication at very high speeds (e.g., at 1.0 GHz andhigher), there are also many applications for lower speed serialcommunication. However, providing low speed analog PLL circuitry can beexpensive in terms of integrated circuit area and other resources.

SUMMARY OF THE INVENTION

In accordance with certain aspects of the invention, the one of aplurality of phase-distributed candidate clock signals that is closestin phase to transitions in another signal (e.g., a CDR signal) isselected by preliminarily selecting two of the candidate clock signalsthat are adjacent to one another in phase and such that at least one ofthe preliminarily selected signals has no other candidate clock signalwith phase between it and the transitions. A final selection is thenmade between the two preliminarily selected signal, but a change in thefinal selection is only allowed while both of the preliminarily selectedsignals have the same polarity.

In accordance with certain other aspects of the invention, apparatus isprovided for selecting from a plurality of phase-distributed candidateclock signals the one of those signals that is closest in phase totransitions in another signal such as a CDR signal. The apparatusincludes preliminary selection circuitry that selects two of thecandidate clock signals that are adjacent to one another in phase, atleast one of these preliminarily selected signals having no othercandidate clock signal with phase between it and the transitions. Theapparatus further includes final selection circuitry that selects theone of the preliminarily selected signals that has phase closer to thetransitions, the final selection circuitry being operable to make achange in selection only when both of the preliminarily selected signalshave the same polarity.

In accordance with still other aspects of the invention, the one of aplurality of candidate recovered clock signals that is closest in phaseto transitions in another signal is selected by preliminarily selectingtwo of the candidate signals that are adjacent to one another in phase,a first of the preliminarily selected signals having phase earlier thanthe transitions, and a second of the preliminarily selecting signalshaving phase later than the transitions. The second preliminarilyselected signal is used to clock a final selection request signalthrough a delay chain, and the final selection request output by thedelay chain is used to make a final selection between the preliminarilyselected signals.

In accordance with yet another aspect of the invention, apparatus isprovided for selecting from a plurality of phase-distributed candidaterecovered clock signals the one of those signals that is closest inphase to transitions in another signal, the apparatus including phasedetect circuitry for comparing the phase of a currently finally selectedone of the candidate recovered clock signals to each of the transitions.The phase detect circuitry produces a first signal if the transition islater than the phase of the currently finally selected signal, and itproduces a second signal if the transition is earlier than the phase ofthe currently finally selected signal. The apparatus further includesdigital integrator circuitry for digitally integrating the first andsecond signals together. The apparatus still further includespreliminary selection circuitry for preliminarily selecting twophase-adjacent ones of the candidate recovered clock signals based onmore significant information from the digital integrator circuitry, andfinal selection circuitry for finally selecting one of the twopreliminarily selected signals based on less significant informationfrom the digital integrator circuitry. And the apparatus includes delaycircuitry for delaying response of the final selection circuitry to theless significant information relative to response of the preliminaryselection circuitry to the concurrently produced, more significantinformation.

In accordance with further aspects of the invention, the operating rangeof circuitry for recovering data information from a serial data signalmay be extended to frequencies below those economically provided byanalog PLL circuitry (or such CDR circuitry may be employed on a serialdata signal having a relatively low bit rate) by selectively dividingthe frequency of each of several candidate clock signals prior to use ofthose candidate clock signals to recover the data information from theserial data signal. On the transmit side, over-transmission orover-sampling may be used for similar reasons (i.e., to effectivelyincrease the bit rate of data having a bit rate lower than frequenciesthat can be economically provided by analog PLL circuitry).

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic block diagram of an illustrativeembodiment of circuitry in accordance with the invention.

FIG. 2 is a more detailed, but still simplified schematic block diagramof an illustrative embodiment of a portion of the FIG. 1 circuitry inaccordance with the invention.

FIG. 3 shows several illustrative signal waveforms that are useful inexplaining certain aspects of the invention. All of these waveforms aredrawn with reference to a common horizontal time-line, along which timeincreases to the right.

FIG. 4 is a simplified block diagram of illustrative, more extensivecircuitry that can include circuitry of the type shown in FIG. 1 inaccordance with the invention.

FIG. 5 is a simplified block diagram of an illustrative system employingcircuitry in accordance with the invention.

FIG. 6 is a simplified schematic block diagram generally similar to FIG.1, but showing an illustrative embodiment of alternative or additionalcircuitry in accordance with the invention.

FIG. 7 is a chart showing illustrative operating modes and ranges ofcircuitry of the type shown in FIG. 6 in accordance with the invention.

FIG. 8 is a simplified schematic block diagram showing an illustrativeembodiment of a representative one of some elements that are employed inFIG. 6 in accordance with the invention.

FIG. 9 is a simplified block diagram of an illustrative embodiment ofanother element that is employed in FIG. 6 in accordance with theinvention.

FIG. 10 shows illustrative, simplified signal waveforms that are usefulin explaining certain aspects of the operation of the element shown inFIG. 9 in accordance with the invention.

FIG. 11 is similar to FIG. 6, but shows another illustrative embodimentof FIG. 6 type circuitry in accordance with the invention.

FIG. 12 is a table showing illustrative modes and ranges of operation ofthe FIG. 11 circuitry in accordance with the invention.

FIG. 13 is a simplified schematic block diagram of an illustrativeembodiment of a representative one of several elements that are used inFIG. 11 in accordance with the invention.

FIG. 14 is a simplified schematic block diagram of circuitry that maycombine aspects of what is shown in FIG. 1 with aspects of what is shownin FIG. 6 or FIG. 11 in accordance with the invention.

FIG. 15 is a simplified schematic block diagram of additional circuitrythat may be employed in accordance with the invention; for example, incircuitry of the type shown in FIG. 14.

DETAILED DESCRIPTION

Illustrative circuitries that may employ DPLL circuitry are shown, forexample, in Aung et al. U.S. Pat. No. 7,227,918 and Lee et al. U.S. Pat.No. 7,366,267. The circuitries shown in these references are alsoexamples of circuitries in which the circuitry of this invention can beemployed. Because these references provide illustrative contexts for thepresent invention, it will not be necessary herein to go into greatdetail about such contexts (although FIGS. 4 and 5 herein and theaccompanying description of those FIGS. do provide some illustrativecontext information). For example, it will be assumed in whatimmediately follows that the input signals to the DPLL circuitry shownherein come from circuitry of the type shown in the references, andsimilarly that the signals output by the DPLL circuitry shown herein areemployed as shown in the references. All context information assumed orprovided herein is only illustrative. Many other contexts are alsopossible.

Turning now to FIG. 1, DPLL circuitry 150 receives serial data (e.g., aCDR signal) via lead 152. DPLL circuitry 150 also receives eightcandidate recovered clock signals via leads 154. These eight candidaterecovered clock signals all have the same frequency, which matches thefrequency of clock information in the CDR signal on lead 152. However,the phases of the eight candidate recovered clock signals on leads 154are all different. Preferably the shift in phase from one of thesesignals to the next is substantially equal to one-eighth of a cycle ofany one of these signals. Thus the phase of each of the signals on leads154 is shifted by 45° from the preceding signal in the group when thesignals are ordered on the basis of phase. In other words, the eightsignals on leads 154 collectively divide one full cycle of any of thesesignals into eight equal fractions. DPLL circuitry 150 operates (as willbe described later herein) to select two of the eight signals on leads154 as final recovered clock signals on leads 192. Omitting for themoment details that will be discussed later, the two signals thusfinally selected are in general the true and complement of the candidatereference clock signal having the phase that best matches the phase ofthe clock information in CDR signal 152. Operation of the FIG. 1circuitry will now be discussed in more detail.

The recovered clock signals on leads 192 are applied as clock signals tophase detect circuitry 160. This circuitry compares the phases oftransitions in CDR signal 152 to phases of the recovered clock signals(from leads 192) and produces “UP” or “DN” signal pulses on leads 161,depending on whether the phase of the recovered clock signals needs tobe delayed (“UP”) or advanced (“DN”) to make the recovered clock signalsbetter match the phase of transitions in CDR signal 152. In circuitry160 the recovered true clock signal may be compared in phase topositive-going transitions in CDR signal 152. The recovered complementclock signal may be compared in phase to negative-going transitions inCDR signal 152.

Circuitry 160 also uses recovered clock signals 192 to produce a retimedserial data signal on lead 200. This may be done, for example, by usingan appropriate one (or a phase-shifted version of an appropriate one) ofthe recovered clock signals to clock CDR signal 152 into a register. Theoutput signal of the register is the retimed serial data signal on lead200.

Circuitry 162 operates as N-to-1 filter circuitry on the UP and DNsignal pulses output by circuitry 160. For example, circuitry 162 maydivide by an integer N (which is greater than 1) the number of UP pulsesreceived to produce a “DIVUP” output signal pulse only after N UP pulseshave been received. Circuitry 162 may do the same thing with respect toDN pulses, so that a “DIVDN” output signal pulse is produced only afterN DN pulses have been received. The DIVUP and DIVDN signals are outputvia leads 163. A purpose of the filtering provided by circuitry 162 isto avoid reacting to the UP/DN signals before phase detect circuitry 160has had a chance to examine the previously chosen phase and make anUP/DN decision based on that phase choice. N to 1 filter circuitry 162is clocked by one of the recovered clock signals from leads 192, andalso by a second clock signal that is one of the recovered clock signals192 after frequency-halving by divide-by-2 circuitry 166. Some functionsin circuitry 162 can be clocked at the full recovered clock rate. Butother functions, such as synthesized counters, need the slower clocksignal produced by divider circuitry 166.

DIVUP and DIVDN signals 163 respectively increment and decrement acounter in phase select circuitry 164. Circuitry 164 is also clocked bya frequency-halved recovered clock signal produced by divider circuitry166. It will be appreciated that elements 162 and 164 effectivelylow-pass-filter and digitally integrate the UP and DN signals together(over time) to help smooth out the response of the circuitry toindications of need to change the phase of the recovered clock signal.It will also be appreciated that, in the particular embodiment beingdescribed, UP and DN signals are only produced in response totransitions in CDR signal 152. In the absence of further transitions inthe CDR signal, the circuitry is therefore stable with respect to anyrecovered clock signal selection that has been made. As a consequence,the illustrative circuitry does not require CDR signal 152 to complywith any particular run length limitation. Run length monitoringcircuitry can be added if desired.

The more significant bits (“MSB”) of the count from the counter incircuitry 164 control “preliminary” selection of two pairs of two of theeight candidate recovered clocks 154. The two candidate recovered clocksignals in each pair have phases that are separated by 45°, and thepairs are 180° out of phase with one another (i.e., the morephase-retarded signal in each pair is 180° out of phase with the morephase-retarded signal in the other pair, and the same is true for morephase-advanced signal in each pair). In the immediately followingdiscussion we will first give primary consideration to only one of theabove-mentioned pairs (i.e., the signals referred to as P1 and P2).Later we will come back and supplement the consideration of the otherpair (i.e., the signals referred to as N1 and N2).

Considering first the selection of signals P1 and P2, these are twophase-adjacent ones of the eight candidate recovered clock signals onleads 154. These two selected signals are “phase-adjacent” because theyhave the smallest possible phase difference)(45° between them. Atvarious times during operation of the circuitry, any two candidaterecovered clock signals, separated in phase by 45°, may be selected asP1 and P2 based on the MSB of the circuitry 164 counter. At any giventime, however, (after the circuitry has been in operation long enough tohave reached reasonable stability) the two candidate recovered clocksignals that are selected as P1 and P2 by the MSB of the circuitry 164counter are the two signals having phases that most nearly match thephase of positive-going transitions in the clock information in CDRsignal 152. This generally means that one of the selected signals willhave phase that is somewhat behind the phase of positive-goingtransitions in the CDR signal clock, and the other selected signal willhave phase that is somewhat ahead of the phase of positive-goingtransitions in the CDR signal clock. In other words, the circuitryattempts to keep the phase of positive-going transitions in the CDRsignal clock information between the phases of the two candidaterecovered clock signals selected as P1 and P2 by the MSB of thecircuitry 164 counter.

Multiplexer circuitry 170 actually makes the selection of the twocandidate recovered clock signals P1 and P2 as described in thepreceding paragraph. Multiplexer circuitry 170 is controlled to makethese selections by SP1[2:0] and SP2[2:0] output signals of phase selectcircuitry 164. These SP1 or SP2 signals are derived from theabove-described MSB information. Although any other consistentconvention could be used, in the illustrative embodiment being describedherein, the higher the value represented by SP1 or SP2, the later (moredelayed) the phase of the candidate recovered clock signal that will beselected in response to that SP1 or SP2 value. As has already beenanticipated, the two signals selected by the SP1 and SP2 signals arerespectively referenced P1 and P2 in FIG. 1.

Only one of the two sets of signals SP1 and SP2 is allowed to change atany one time. For example, if SP1 and SP2 are selecting candidaterecovered clock signals P1 and P2 having phases that are respectivelybehind and ahead of the CDR signal clock information phase, and if it isthen found that the phase of P2 is now also behind the phase of the CDRsignal clock information, SP2 (and therefore P2) does not change. OnlySP1 (and therefore P1) changes. In particular, the change in SP1 is fromselecting the candidate signal having phase behind the phase of P2 toselecting the candidate signal having phase ahead of the phase of P2. Inthis way the phase of the CDR clock information remains between thephases of P1 and P2, but only one signal selection (in this example theselection of P1) changes at any one time. Thus there is always one setof signals SP1 or SP2 that is unchanged during any change in the otherset of the SP1/SP2 signals. By the same token, there is always one ofsignals P1 or P2 that is uninterruptedly output by circuitry 170 duringany change in the other P1/P2 signal output by that circuitry.

At the same time that multiplexer circuitry 170 is selecting P1 and P2as described above, that circuitry also selects the complements of P1and P2 (referred to as N1 and N2, respectively). (All “complement”signals are 180° out of phase with the corresponding “true” signal.) TheSN1[2:0] and SN2[2:0] output signals of phase select circuitry 164control circuitry 170 to make these complement signal selections.

A final selection of one of signals P1 and P2 for use as the recoveredclock signal is made by so-called digital interpolator circuitry 182.Within circuitry 182, circuitry 184 compares the SP1 and SP2information. If SP2 is greater than SP1, the phase of P2 is later (moreretarded or delayed) than the phase of P1. In that case comparecircuitry 184 causes multiplexer circuitry 186 to select P2 forapplication to the clock input terminal of register chain 188. On theother hand, if SP1 is greater than SP2, the phase of P1 is later (moreretarded or delayed) than the phase of P2. In that case comparecircuitry 184 causes multiplexer circuitry 186 to select P1 forapplication to the clock input terminal of register chain 188. From theforegoing it will be seen that multiplexer circuitry 186 always outputsthe one of signals P1 and P2 with the later phase. It will also now beappreciated why it is desirable for only one set of signals SP1 or SP2to be allowed to change at any one time (e.g., to increase thereliability of operation of comparison circuitry 184).

The data input to register chain 188 is a less significant bit (“LSB”)of the count in the above-described counter in phase select circuitry164. The LSB information can be the least significant bit of the countin the circuitry 164 counter, or if there are several bits in thatcounter with significance less than the previously described MSBinformation, the LSB can be one of those less significant bits(preferably the bit with significance just less than the MSBinformation). The LSB signal propagates through register chain 188 atthe rate of the clock signal (P1 or P2) applied to the clock input ofthat chain from the output of multiplexer 186. After thus propagatingthrough register chain 188, the LSB signal information is output by thatchain as final selection signal SEL. The SEL signal is used to controlmultiplexer circuitry 190 to select either P1 and N1 or P2 and N2 as therecovered clock signal and its complement. In particular, if SEL is 0,circuitry 190 selects P1 and N1 for application to leads 192. If SEL is1, circuitry 190 selects P2 and N2 for application to leads 192.

Reviewing the operation of the circuitry from a relatively high level,the phase of the clock signal applied to register chain 188 can changeby no more than 45° at any one time. This helps register chain 188continue to operate satisfactorily during any change in the signalselected by multiplexer 186. Register chain 188 delays the time betweenany change in the LSB information and the use of that information (asSEL) to cause a change in the selection of P1/N1 or P2/N2 forapplication to leads 192. If the MSB and LSB information both change atthe same time, the delay in use of the LSB information that results frompassing that information through register chain 188 prior to use tocontrol multiplexer 190 prevents a change in candidate clock signalselection by multiplexer 190 from occurring too close in time to achange in candidate clock signal selection by more upstream multiplexers170 and 186. This means that any change in (“preliminary”) selections bymultiplexers 170 and 186 has been made and the results of thoseselections have been well stabilized before any change in further(“final”) selection among those preliminary selections can be attemptedand made by multiplexer 190. Ensuring in this way that the initial orpreliminary selections (by multiplexers 170 and 186) and the finalselections (by multiplexer 190) are well spaced apart in time helpsensure that the final selections (the recovered clock signals on leads192) are free of “glitches”, even when those final selections change, asthey typically do at least from time to time. After the immediatelyfollowing additional point, further glitch-preventing aspects of thecircuitry will be discussed in connection with FIG. 2.

Before leaving FIG. 1, it should be pointed out (if it is not alreadyapparent from what has been said) that whenever a change is made in thepreliminary selections by multiplexer circuitry 170, the LSB informationwill typically already be causing multiplexer circuitry 190 to finallyselect the signals P1/N1 or P2/N2 that will not change as a result ofthe preliminary selection change. This is so because (as has been said)only one of the two sets of signals preliminarily selected bymultiplexer circuitry 170 is allowed to change at any one time.Moreover, the set that is allowed to change is the set that is moredistant in phase from transitions in CDR signal 152. But before thatpreliminary selection change occurs, the LSB/SEL information will havecaused the final selection (via operation of multiplexer circuitry 190)to be selection of the preliminarily selected set that is closer inphase to transitions in CDR signal 152. So, although a change inpreliminary selection is immediately reflected at one set of the inputsto multiplexer circuitry 190, that has no immediate effect on theoutputs of circuitry 190 because SEL is then causing circuitry 190 toderive its outputs from its other set of inputs. And there is no changein the signals applied to that other set of circuitry 190 inputs. Onlywell after a change in one of the sets of inputs to circuitry 190 canSEL change to cause final selection of that changed set of inputs. Thisis ensured by operation of delay circuitry 188, which delays any changein LSB prior to appearance of that change in SEL. This description, ofcourse, assumes normal operation of the circuitry.

Turning now to FIG. 2, that FIG. shows an illustrative embodiment ofdigital interpolator circuitry 182 in somewhat more detail. The portionsof this circuitry that relate to selection of OUTP (the finally selectedrecovered clock on lead 192 a) will be discussed first. Then the similarcircuitry for selecting the complement of OUTP (i.e., OUTN on lead 192b) will be discussed.

As has already been said, the final selection between P1 and P2 iscontrolled by LSB. An LSB transition should not cause a glitch in theoutput clock OUTP (or OUTN). To prevent such a glitch, LSB is forced totransition at the final output mux 190 a only when P1 and P2 are thesame logic state (high or low). (The same is true for N1 and N2, i.e.,they are forced to be in the same logic state when LSB transitions atfinal output mux 190 b.)

First, SP1 and SP2 are compared in circuitry 184, and the later phaseclock among P1 and P2 is selected to register LSB. For example, if SP1=2and SP2=1, then the phase of P1 is greater (later) than the phase of P2and therefore P1 is selected to register LSB.

The waveforms in FIG. 3 illustrate the glitch-free operation of thecircuit in FIG. 2. The “safe window” in FIG. 3 shows the area where SELis guaranteed to switch. As can be seen from FIG. 3, P1=P2 and N1=N2 inthis safe window. This ensures that SEL changing state and causing achange in the sources of output clocks OUTP and OUTN does not cause aglitch in OUTP and OUTN (because both possible sources for each of thosesignals then have the same logic level).

Another characteristic of the circuitry that helps to ensure glitch-freeoperation is that the TCO (time from clock to output) of shift registerstage 188 d 1 (FIG. 2) plus TSEL (time from a change in SEL to a changein the output of the multiplexers 190 a and 190 b controlled by SEL) isless than the “safe window” Also, the dummy loads (shown in dashed linesin FIG. 2 (e.g., multiplexers 186 b-f, registers 188 d 2-5, and buffers189 b-e)) help to match the delays between the five clocks (i.e., P1,P2, N1, N2, and the clock to the fourth register 188 d 1 in registerchain 188 a-188 d 1).

Illustrative circuitry 500 in which DPLL circuitry as described abovecan be used in accordance with the invention is shown in FIG. 4.Circuitry 500 is assumed to be programmable logic device (“PLD”)circuitry. Phase locked loop (“PLL”) circuitry 510 receives a referenceclock signal having frequency related to the frequency of the clockinformation in the CDR signal 152 also received by circuitry 500. Forexample, the reference clock signal frequency can be the same as thefrequency of the CDR signal clock information, or there can be aninteger multiple relationship between these two frequencies. PLL 510produces the above-described eight candidate recovered clock signals onleads 154. DPLL 150 uses the signals on leads 154 and CDR signal 152 toproduce final recovered clock signal(s) 192 and retimed data signal 200.These signals may be used and/or further processed in other interfacecircuitry 520, and/or they may be applied to PLD core circuitry 530(e.g., the general-purpose programmable logic circuitry of PLD 500).Examples of other interface circuitry 520 that may be included are (1)byte alignment circuitry, (2) 8-bit/10-bit decoding circuitry, (3)channel de-skew circuitry, (4) byte de-serializer circuitry, (5)decryption circuitry, etc. Examples of such possible other interfacecircuitry 520 are shown in such references as Aung et al. U.S. Pat. No.7,227,918, Lee et al. U.S. Pat. No. 7,366,267, Lee et al. U.S. Pat. No.6,650,140, Venkata et al. U.S. Pat. No. 6,750,675, Venkata et al. U.S.Pat. No. 7,180,972, Venkata et al. U.S. Pat. No. 6,854,044, and Venkataet al. U.S. Pat. No. 7,305,058. Other interface circuitry 520 mayexchange signals with PLD core circuitry 530 via leads 522 and 524. Forexample, signals for controlling certain operations of circuitry 520 maycome from PLD core circuitry 530 via leads 524. Further processed datasignals and/or signals indicating the status of various aspects ofcircuitry 520 operation may be applied to circuitry 530 via leads 522.There may be still other connections (not shown) between variouselements in FIG. 4. For example, PLL 510 and/or DPLL 150 may indicate“loss of lock” to circuitry 530. As another example, DPLL 150 mayinclude circuitry for monitoring the run length of CDR signal 152 andmay indicate any “run length violation” to circuitry 530.

FIG. 5 illustrates a PLD or other circuitry 500 like that illustrated byFIG. 4 in a data processing system 602 in accordance with the invention.Data processing system 602 may include one or more of the followingcomponents: a processor 604; memory 606; I/O circuitry 608; andperipheral devices 610. These components are coupled together by asystem bus or other interconnections 620 and are populated on a circuitboard 630 (e.g., a printed circuit board), which is contained in anend-user system 640. Any of the interconnections between element 500 andany other elements may be made using the above-described CDR signaling.

System 602 can be used in a wide variety of applications, such ascomputer networking, data networking, instrumentation, video processing,digital signal processing, or any other application where the advantageof using programmable or reprogrammable logic is desirable. Circuitry500 can be used to perform a variety of different logic functions. Forexample, circuitry 500 can be configured as a processor or controllerthat works in cooperation with processor 604. Circuitry 500 may also beused as an arbiter for arbitrating access to a shared resource in system602. In yet another example, circuitry 500 can be configured as aninterface between processor 604 and one of the other components insystem 602. It should be noted that system 602 is only exemplary, andthat the true scope and spirit of the invention should be indicated bythe following claims.

In the embodiments shown and described thus far it is assumed that theserial data rate is high enough that PLL 510 (FIG. 4) can economicallyoperate at that frequency. However, there are many possible applicationsof CDR technology in which the serial data rates are lower than analogCDR circuitry (e.g., including analog PLL circuitry 510) caneconomically support. For example, providing analog CDR circuitry thatcan operate at low frequencies (e.g., below about 0.5 GHz (although thisnumber can be more or less, depending on circuit configuration)) may beprohibitively expensive in integrated circuit area and powerrequirements. FIG. 6 shows an illustrative embodiment of the inventionin which the operating range of circuitry of the type discussed hereincan be digitally extended at the lower end. This digital low-rangeextension for analog CDR circuitry avoids the need for analog circuitrythat can operate at relatively low frequencies and that may, as aconsequence, be expensive to provide in terms of the types of resourcesmentioned earlier in this paragraph.

The circuitry shown in FIG. 6 has many similarities in terms ofconstruction and operation to the circuitry shown in FIG. 1. Elements inFIG. 6 that are similar to elements in FIG. 1 have FIG. 6 referencenumbers that are increased by 1000 relative to the corresponding FIG. 1element reference numbers. Because of the similarity of what is shown inFIG. 6 to what is shown in FIG. 1 and fully described earlier in thisspecification, it will not be necessary to describe all aspects of FIG.6. For the most part, only respects in which FIG. 6 differs from FIG. 1will be described. It will be understood that other aspects of FIG. 6are covered by the description of corresponding aspects of FIG. 1.

Also in conjunction with FIG. 6, FIG. 7 should be considered. FIG. 7 isa table showing certain aspects of the operation of FIG. 6 in two modesof such operation. The first of these modes is called “normal” mode. Thesecond mode is called “low” mode. Both of these modes support serialdata rates that can be lower than the lowest serial data rate supportedby similarly constructed circuitry as shown in FIG. 1. FIG. 7 assumesthat the lowest analog CDR (A-CDR or ACDR) serial clock frequencypermitted is 0.5 GHz (500 MHz). In the normal mode shown in FIG. 7 theserial data rate can be anywhere from 0.25 Gpbs to 1.0 Gpbs. This modeuses a serial clock frequency that is twice the serial data rate (i.e.,anywhere from 0.5 GHz to 2.0 GHz). In the low mode shown in FIG. 7 theserial data rate can be anywhere from 0.1 Gpbs to 0.25 Gbps. This modeuses a serial clock frequency that is eight times the serial data rate(i.e., anywhere from 0.8 GHz to 2.0 GHz). It will thus be seen thatthese modes keep the serial clock frequency at or above 0.5 GHz, butallow the serial data rate to be significantly lower than 0.5 Gpbs, ifthat is desired. It should be noted that in both normal mode and lowmode the ratio between the serial bit rate and the “DPLL 8-phase clock”frequency is always 1 to 2. In other words, the “DPLL 8-phase clock”frequency is always twice the serial bit rate.

Returning to FIG. 6, in this embodiment instead of operating with eightcandidate clock signals (with 45° phase spacing) from ACDR circuitry 510(FIG. 4) as in FIG. 1, four candidate clock signals 1154 (with 90° phasespacing) from ACDR circuitry 510 are used. If normal mode is desired,these four candidate clock signals pass through multiplexer circuitry1153 to leads 1154′ with no change in frequency. If low mode is desired,multiplexer circuitry 1153 is switched to apply the outputs of element1151 to leads 1154′. Element 1151 divides the frequency of each of thefour candidate clock signals by four. The selection control lead ofmultiplexer circuitry 1153 may be controlled by a programmable element1155 (e.g., a RAM cell) associated with the circuitry. The remainder ofthe discussion of FIG. 6 (and its related FIGS.) is the same whether theclock signals on leads 1154′ have the same frequency as the signals onleads 1154 or one-quarter the frequency of the signals on leads 1154.

FIG. 8 shows a representative one of two 8 to 1 multiplexers 1170 inFIG. 6 in more detail. This multiplexer is the one that uses theSP1[2:0] signals from phase select circuitry 1164 (FIG. 6) to selectfurther candidate clock signal P1 from candidate clock signals 1154′.FIG. 8 shows the four signals 1154′ with respective relative phases 0°,90°, 180°, and 270°. FIG. 8 shows that these four signals 1154′ are eachused twice (i.e., by each being applied to two input terminals ofmultiplexer 1170). Moreover, the second application of each signal tomultiplexer 1170 is at what is effectively 360° phase spacing from thefirst application. Thus the phasing of the second applications of the0°, 90°, 180°, and 270° signals looks like phasing of 360°, 450°, 540°,and 630°, respectively. Multiplexer 1170 selects the 0° signal to outputas P1 when the SP1[2:0] signals are 000 (or 100). Multiplexer 1170selects the 90° signal to output as P1 when the SP1[2:0] signals are 001(or 101). Other selections made by multiplexer 1170 will be apparentfrom these examples.

The construction and operation of the multiplexer 1170 that iscontrolled by the SP2[2:0] signals to select further candidate clocksignal P2 can be similar to what has been described for the multiplexercontrolled by the SP1[2:0] signals. (Although FIG. 6 shows SN1[2:0] andSN2[2:0] signals also being generated in circuitry 1164, these signalsare not actually used in this embodiment.)

An illustrative embodiment of phase detector 1160 (FIG. 6) is shown inFIG. 9. Divide by 2 circuitry 1210 receives the clock signal output bydigital interpolator circuitry 1182 and divides the frequency of thatclock signal by 2. The output signal of circuitry 1210 is recoveredclock signal 1192. The clock signal applied to circuitry 1210 is alsoapplied as a clock signal to registers 1220, 1222, and 1224, and to ATBlogic 1230. Register 1220, 1222, and 1224 are also known as registers A,T, and B, respectively. The serial data signal is applied to each ofregisters 1220, 1222, and 1224, and is accordingly sampled at eachpositive-going transition in the clock signal applied to thoseregisters.

FIG. 10 shows illustrative sampling of the serial data in circuitry1160. The A sample in FIG. 10 is derived from register 1220 in FIG. 9.The T sample in FIG. 10 is derived from register 1222 in FIG. 9. The Bsample in FIG. 10 is derived from register 1224 in FIG. 9. Each ofregisters 1220-1224 in FIG. 9 actually takes more samples than arehighlighted by the letters A, T, and B in FIG. 10, but ATB logic 1230(FIG. 9) generally considers only every fourth sample taken by each ofthese registers. In particular, ATB logic 1230 only considers sampleslike sample A from register 1220, samples like sample T from register1222, and samples like sample B from register 1224. Samples like A maybe thought of as “ahead” (in phase) of positive-going transitions in theserial data signal. Samples like B may be thought of as “behind” (inphase) positive-going transitions in the serial data signal. Sampleslike T may be thought of as (approximately) at or concurrent withpositive-going transitions in the serial data signal.

ATB logic 1230 (FIG. 9) examines each group of samples like A, T, and Bin FIG. 10 to determine whether to output an UP or DN output signalpulse. ATB logic 1230 can operate in many different ways (and can haveany of many different configurations appropriate to the desiredoperation), but the following will be given as one possible example.First, ATB logic 1230 looks for three consecutive samples in whichsample A (from register 1220 (FIG. 9)) is low and sample B (fromregister 1224 (FIG. 9)) is high. When ATB logic 1230 finds samples A andB satisfying this criterion, the ATB logic then examines the level ofthe intervening sample T (from register 1222 (FIG. 9)). If sample T islow, it was taken ahead of or earlier than the positive-going transitionin the serial data signal, and an UP signal is output to indicate a needto increase the phase angle of the sampling clock signal. If sample T ishigh, it was taken behind or later than the positive-going transition inthe serial data signal, and a DN signal is output to indicate a need todecrease the phase angle of the sampling clock signal.

FIG. 9 also shows the output of B register 1224 being sampled insynchronism with the recovered clock signal by further registercircuitry 1240. The output signal of register 1240 is retimed serialdata signal 1200.

Returning to FIG. 6, comparison of that FIG. with FIG. 1 reveals thedifferences that will now be discussed (in addition to thealready-discussed differences in the clock signals going intomultiplexers 1170). Wherever in FIG. 1 the circuitry uses the output ofmux 190, the FIG. 6 circuitry uses the comparable signal (output by mux1190) after frequency division by 2. In phase detector 1160 thisfrequency division is performed by circuitry 1210 (FIG. 9). To theleft-hand clock input to circuitry 1162 this frequency division isperformed by circuitry 1167. Similarly, wherever in FIG. 1 the circuitryuses the output of mux 1190 after frequency division by 2, the FIG. 6circuitry uses the comparable signal after frequency division by 4 (asperformed by circuitry 1169).

Another difference between the FIG. 1 and FIG. 6 circuitries is thatwhereas in FIG. 1 multiplexer circuitry 190 selects and outputs bothtrue and complement clock signals 192, in FIG. 6 multiplexer circuitry1190 selects and outputs only one (true) clock signal.

The last-mentioned difference between FIGS. 1 and 6 should perhaps beamplified as follows. Because the FIG. 1 circuitry outputs frommultiplexer circuitry 190 both true and complement clock signalselections 1192, clock signals in this type of circuitry must have a 50%duty cycle. This requirement is substantially relaxed in circuitry ofthe type shown in FIG. 6, in which (effectively) only a true clocksignal is output by multiplexer circuitry 1190. Relaxing the FIG. 1requirement for 50% duty cycle clocks simplifies the FIG. 6 circuitryrelative to FIG. 1. Selecting (at mux 1190) and using (e.g., in phasedetector 1160) only one (true) clock signal also simplifies the FIG. 6circuitry relative to FIG. 1.

Except for the differences discussed above, the FIG. 6 circuitryoperates in the general manner described above in connection withFIG. 1. The applicable portions of the FIG. 1 discussion will thereforenot need to be repeated here for FIG. 6. It will be sufficient to sum upthe discussion of FIG. 6 by reiterating that this embodiment allows theserial data rate (signal on lead 1152) to be either one-half orone-eighth the ACDR clock signal frequency (signals on leads 1154). (Theone-half relationship applies when circuitry 1151 is not in use, and theone-eighth relationship applies when circuitry 1151 is in use.) This canhelp avoid the need for low-speed ACDR clock circuitry when it isdesired to work with relatively low serial data rates.

FIG. 11 shows an alternative embodiment that also allows the serial datarate to be a fraction of the A-CDR clock frequency if desired, and thatalso decreases the angle of jitter as compared to the FIG. 6 embodiment.FIG. 12 shows the various operating modes and ranges of the FIG. 11circuitry. As in the case of FIG. 6, the FIG. 11 circuitry has twopossible modes: normal and low. In normal mode the serial data rate isone-quarter the A-CDR clock frequency. Thus with an A-CDR clockfrequency as low as 0.8 GHz, the serial data rate can be as low as 0.2Gbps. In low mode the serial data rate is one-eighth the A-CDR clockfrequency. Accordingly, in low mode the serial data rate can be as lowas 0.1 Gbps with an A-CDR clock frequency as low as 0.8 GHz.

The circuitry shown in FIG. 11 has many similarities to the FIG. 6circuitry. Accordingly, the reference numbers used for various elementsin FIG. 11 that are similar to elements in FIG. 6 have reference numbersin FIG. 11 that are increased by 1000 relative to the reference numbersof the corresponding elements in FIG. 6. Thus, for example, N to 1filter 2162 in FIG. 11 corresponds to N to 1 filter 1162 in FIG. 6. Thisbasic similarity between almost all elements in FIGS. 6 and 11 will makeit unnecessary to again describe many aspects of many elements in FIG.11.

Like the FIG. 6 circuitry, the FIG. 11 circuitry receives four candidateclock signals 2154—spaced 90° apart—from A-CDR circuitry such ascircuitry 510 in FIG. 4. Multiplexer circuitry 2153 passes those signalsto leads 2154′, either unaltered (normal mode) or after frequencydivision by 2 in divide by 2 circuitry 2151 (low mode). Multiplexercircuitry 2153 may be controlled to make this selection by programmablememory (RAM) cell 2155.

Only two of the output signals of multiplexer circuitry 2153 areactually used, and the two signals that are used are spaced apart inphase by 180°. FIG. 13 shows application of these two signals to arepresentative one of two 16 to 1 multiplexers 2170. The multiplexershown in FIG. 13 is the one that uses the SP1[3:0] signals output byphase select circuitry 2164 (FIG. 11) to select the further candidateclock signal P1. FIG. 13 shows that the 0° phase candidate clock signal2154′ is applied to the “even” numbered inputs to the depictedmultiplexer 2170, while the 180° phase candidate clock signal 2154′ isapplied to the “odd” numbered inputs to that multiplexer. Thus depictedmultiplexer 2170 outputs the 0° phase signal 2154′ when the SP1[3:0]signals represent the binary equivalent of decimal 0, 2, 4, 6, 8, 10,12, or 14, and it outputs the 180° phase signal 2154′ when the SP1[3:0]signals represent the binary equivalent of decimal 1, 3, 5, 7, 9, 11,13, or 15. The other one of multiplexers 2170 in FIG. 11 is similar towhat is shown in FIG. 13, except that it uses the SP2[3:0] signals toselect the P2 signal from the same inputs as are shown for themultiplexer in FIG. 13.

Because the output signal of multiplexer 2190 in FIG. 11 is one or theother of above-described signals P1 or P2, that signal has frequencythat is four times the serial data rate of signal 2152. This is truewithout regard for whether the circuitry is operating in normal or lowmode. (As noted above, in low mode the A-CDR output signals on leads2154 have frequency eight times the serial data rate, but that ratio isreduced to four by frequency divider 2151.) Accordingly, with theaddition of frequency divider 2165 in FIG. 11, phase detect circuitry2160 can be constructed and operated as shown (in FIGS. 9 and 10) anddescribed above for phase detect circuitry 1160. Divide by 2 circuitry2165 needs to be added to the circuitry of FIG. 11 (as compared to thecircuitry of FIG. 6) because the ratio between the frequency of theoutput signal of multiplexer 2190 and the serial data rate is four (ascompared to a corresponding ratio of two in the FIG. 6 embodiment).

In the same way that frequency division needs to be increased by afactor of two in going from FIG. 6 to FIG. 11 at the point (2165)described in the preceding paragraph, other frequency divisions (1167and 1169) in FIG. 6 need to be increased by a factor of two for the FIG.11 embodiment. Thus divide by 2 circuitry 1167 in FIG. 6 becomes divideby 4 circuitry 2167 in FIG. 11, and divide by 4 circuitry in FIG. 6becomes divide by 8 circuitry 2169 in FIG. 11.

Another difference between the FIG. 6 and FIG. 11 embodiments is that inFIG. 11 the outputs of phase select circuitry 2164 like SP1[3:0] areeach four bits in length, whereas in FIG. 6 they are only three bits inlength. Four bits are needed in FIG. 11 because in this embodimentmultiplexers 2170 are 16 to 1. In FIG. 6 corresponding multiplexers 1170are only 8 to 1. This difference is also what reduces the amount ofrecovered clock jitter in the FIG. 11 embodiment relative to the amountassociated with the FIG. 6 embodiment. In FIG. 6 there are eightcandidate clock signals 1154′ for the circuitry to choose from. Themagnitude of recovered clock jitter is therefore 360°÷8=45°. In FIG. 11,however, there are 16 candidate clock signals 2154′ for the circuitry tochoose from. The magnitude of recovered clock jitter is therefore360°÷16=22.5°.

Except for the differences mentioned above, the circuitry of FIG. 11 canbe like the FIG. 6 circuitry in construction and operation. Furtherdetailed discussion specific to FIG. 11 is therefore unnecessary.

FIG. 14 shows how circuitry of the type shown in FIG. 1 can be combinedwith circuitry of the type shown in FIG. 6 or FIG. 11 to providecircuitry that can support a wider range of data rates (particularly atthe low end) without having to provide lower-speed PLL clock signals.FIG. 14 also shows transmit as well as receive aspects of suchcircuitry. The circuitry shown in FIG. 14 includes high range physicalmedium attachment (“PMA”) circuitry 3010, low range PMA circuitry 3030,and multiplexer circuitry 3050. High range PMA circuitry 3010 caninclude circuitry like that shown in FIG. 1 with the addition of

(1) deserializer circuitry for converting the retimed serial data outputof the FIG. 1 circuitry to parallel form on leads 3012, and (2)serializer circuitry for converting data to be transmitted from parallelform on leads 3014 to serial form on lead(s) 3016. (Examples of this,including examples of serializer and deserializer circuitry, are shownin Lee et al. U.S. Pat. No. 7,366,267 and some of the other referencesmentioned above.) FIG. 14 shows the circuitry able to support serialdata in either single-ended (e.g., true-only) or double-ended (e.g.,true and complement) form on leads such as 3016 and 3018; and it showsthe circuitry able to support parallel data in any of widths 8 bits, 10bits, 16 bits, or 20 bits on leads such as 3014 and 3052. FIG. 14 showshigh range PMA 3010 being used for data rates in the range from 1.0 Gbpsto 6.5 Gbps; and it shows low range PMA 3030 being used for data ratesin the range from 0.1 Gbps to 1.0 Gbps.

Low range PMA circuitry 3030 can include circuitry of the type shown inFIG. 6 or FIG. 11. In addition, low range PMA circuitry 3030 can include

(1) deserializer circuitry for converting the retimed serial data outputsignal of the FIG. 6 or FIG. 11 circuitry to parallel form on leads3032, and(2) serializer circuitry for converting data to be transmitted fromparallel form on leads 3014 to serial form on lead 3034. Low range PMAcircuitry 3030 receives candidate clock signals (e.g., as on leads 1154in FIG. 6 or 2154 in FIG. 11) and the received serial data (ultimatelyfrom lead(s) 3018) via leads 3020. The serial data output by low rangePMA circuitry 3030 is ultimately output via lead(s) 3016. Multiplexercircuitry 3050 selects either the parallel data output signals 3012 ofhigh range PMA 3010 or the similar signals 3032 of low range PMA 3030 asthe ultimate parallel data outputs 3052 of the circuitry. Low range PMAcircuitry 3030 is only used when the data rate is lower than can behandled by high range PMA 3010.

Whereas the transmitter portion of high range PMA circuitry 3010 can bequite straight-forward (e.g., primarily a serializer), the transmitterportion of low range PMA circuitry 3030 may be somewhat more complicatedto facilitate the transmission of data at relatively low data rateswithout the need for correspondingly low-frequency PLL clocks. Anillustrative embodiment of such low data rate transmitter circuitry isshown in FIG. 15.

The FIG. 15 circuitry includes “gear box” circuitry 3060, registercircuitry 3070, multiplexer circuitry 3080, serializer circuitry 3090,and frequency divider circuitry 3100. Elements 3060, 3070, and 3100 canbe in low range PMA circuitry 3030. Elements 3080 and 3090 can be inhigh range PMA circuitry 3010. The clock signal applied to dividercircuitry 3100 can be a PLL clock signal from circuitry 3010 and henceis shown on a lead 3020. It will be understood, however, that this clocksignal may already have been divided in frequency by a factor such as10. The parallel output signals of register circuitry 3070 are shown onmultiple leads 3034. This is somewhat different from the abovediscussion of FIG. 14, which suggests that circuitry 3030 deserializestransmit data down to a single lead 3034. However, those skilled in theart will understand that what is now described is an alternative to thatearlier discussion. FIG. 15 also differs from FIG. 14 by showingparallel data widths of only 8 bits or 10 bits. Further serializing(so-called byte serialization) may be performed upstream from what isshown in FIG. 15 to increase the number of parallel data widthssupported (e.g., to include 16 bits and 20 bits).

The purpose of gear box circuitry 3060 and register circuitry 3070 is tocontrollably replicate each bit in the parallel data from leads 3014onto a plurality of further parallel leads 3034. For example, if tenbits of data are applied in parallel via leads 3014, gear box 3060outputs each of those bits one after another on lead 3062 at a bit ratethat is 10 times the parallel data byte rate. In this example, if theparallel data byte rate is 10 MHz, then the serial data bit rate is 100MHz. Divider circuitry 3100 divides the clock signal on lead 3020 (whichis at the serial data bit rate needed for the data on lead 3062) to theparallel data byte rate on lead 3102. Gear box circuitry 3060 receivesboth bit rate clock signal 3020 and byte rate clock signal 3102 so thatit can perform the above-described parallel to serial data conversion.

Register circuitry 3070 registers each bit output by gear box 3060 in aplurality of registers. In the illustrative embodiment shown in FIG. 15,register circuitry 3070 includes ten registers that register each bitfrom gear box 3060 in parallel. Register circuitry 3070 is thereforealso clocked by the bit rate clock on lead 3020. The contents of theregisters in circuitry 3070 are applied in parallel to parallel leads3034.

Assuming that circuitry 3060/3070 is in use, multiplexer circuitry 3080applies the signals on as many of leads 3034 as are desired to acorresponding number of leads 3082. For example, all ten register 3070outputs 3034 may be applied to ten leads 3082, or only eight outputs3034 may be applied to eight leads 3082. If circuitry 3060/3070 is notin use, multiplexer 3080 applies the signals on leads 3014 to acorresponding number of leads 3082.

Serializer circuitry 3090 converts the data it receives in parallel vialeads 3082 to serial form on lead 3092. For example, if multiplexer 3090receives 10 bits of parallel data at a byte rate of 100 MHz, it outputsthat data at a serial bit rate of 1000 MHz=1 GHz. If the parallel dataon leads 3082 has come from leads 3034, all of the bits serializer 3090receives at any one time will be the same. The serial output 3092 ofthis data will look like one bit of relatively long duration. Thus therelatively low-speed data passed through elements 3060, 3070, 3080, and3090 is effectively “over-sampled” in order to output it on lead 3092using a PLL clock having a higher frequency than the bit rate of thedata being output. On the other hand, if elements 3060 and 3070 are notbeing used (because the transmit data rate is more compatible with thePLL clock frequency), the various bits received by serializer 3090 atany given time can have different values, and they will each be sampledand output once in serial form on lead 3092.

Some examples of use of circuitry of the type shown in FIG. 15(especially routing through elements 3060 and 3070) will now beprovided.

Example 1

-   -   Data rate: 100 Mbps.    -   Over-transmission rate: 10 times.    -   TX PLL serial clock: 1 GHz (assumed to be the minimum).    -   Parallel data width (leads 3014): 10 bits.    -   Parallel byte rate: 10 MHz.    -   Serial bit rate (lead 3062): 100 MHz.    -   Number of leads 3034 used: 10.    -   Clock frequency on lead 3020: 100 MHz.    -   Clock frequency on lead 3102: 10 MHz.    -   Clock frequency on parallel side of serializer 3090: 100 MHz.    -   Clock frequency on serial side of serializer 3090: 1 GHz.

Assuming that the permissible range for the TX PLL clock is 1 GHz to 6.5GHz (or 0.5 GHz to 3.25 GHz for double data rate (“DDR”) or dual edgeclocking), the general configuration of the circuitry employed in thisexample can be utilized until the serial clock reaches 6.5 GHz (or 3.25GHz DDR). At that point the data rate is 650 Mbps and the clockfrequency on lead 3102 is 65 MHz. After this upper limit is reached, adifferent mode has to be entered where each data bit is over-transmittedeight times. The following Example 2 is an example of this mode.

Example 2

-   -   Data rate: 800 Mbps.    -   Over-transmission rate: 8 times.    -   Clock frequency on lead 3102: 80 MHz.    -   Parallel data width (leads 3014): 10 bits.    -   Serial bit rate (lead 3062): 640 bits.    -   Number of leads 3034 used: 8.    -   Clock frequency on parallel side of serializer 3090: 640 MHz.    -   Clock frequency on serial side of serializer 3090: 2.56 GHz (or        1.28 GHz DDR).

Again assuming that the upper limit for the TX PLL clock is 6.5 GHz(3.25 GHz DDR), the upper limit for operation in the mode illustrated byExample 2 is reached when the data rate is 6.5/8/8=1.015625 Gbps(approximately 1 Gbps).

Two ratios are provided in gear box circuitry 3060 (10 to 1 and 8 to 1)to accommodate the standard data bus widths of 8 and 10.

It will be understood that the foregoing is only illustrative of theprinciples of the invention, and that various modifications can be madeby those skilled in the art without departing from the scope and spiritof the invention. For example, the number of registers 188 a-d 1 inregister chain 188 can be varied (e.g., increased from four) if desired.As another example of possible modifications, the use of eight candidaterecovered clock signals 154 is only illustrative, and any othersuitable, larger or smaller, plural number of such signals can be usedinstead if desired. Other examples of modifications within the scope ofthe invention include (1) analog PLL (A-CDR) clocks with differentfrequency ranges, (2) the use of data buses having different widths, and(3) the use of different frequency division factors or ratios. Any ofthe circuitry shown in FIGS. 6-15 can be used in the circuitry orsystems shown in FIGS. 4 and 5.

1. Apparatus for converting parallel data to serial data comprising:first serializer circuitry operable to receive the parallel data andoperable to output each bit of the parallel data, one bit after anotherin succession; buffer circuitry operable to receive each bit, one bitafter another, being output by the first serializer and, for each bit,the buffer circuitry is operable to output more than two instances ofthat bit in parallel on a plurality of more than two parallel leads; andsecond serializer circuitry operable to receive signals from theplurality of more than two parallel leads in parallel, and wherein saidsecond serializer circuitry is operable to output each of those signals,one signal after another, on a serial output lead.
 2. The apparatusdefined in claim 1 wherein width of the parallel data is selectable. 3.The apparatus defined in claim 1 wherein number of leads in theplurality of more than two parallel leads is selectable.
 4. Theapparatus defined in claim 1 further comprising: routing circuitryoperable to selectively substitute the parallel data for the outputs ofthe first serializer circuitry on the plurality of more than twoparallel leads.
 5. The apparatus defined in claim 1 wherein the firstserializer circuitry, the buffer circuitry, and the second serializercircuitry are coupled in series with one another, in order, from thefirst serializer circuitry through the buffer circuitry to the secondserializer circuitry.
 6. A method for converting parallel data to serialdata comprising: generating a first set of serial data, the first set ofserial data comprising each bit of a first set of parallel data arrangedone bit after another in succession; generating a second set of paralleldata, the second set of parallel data comprising more than twoinstances, arranged in parallel, of each successive bit of the first setof serial data; and generating a second set of serial data, the secondset of serial data comprising each bit of the second set of paralleldata arranged one bit after another in succession.
 7. The method definedclaim 6, wherein width of the first set of parallel data is selectable.8. The method defined in claim 6, wherein width of the second set ofparallel data is selectable.
 9. Apparatus for converting parallel datato serial data comprising: first serializer circuitry operable togenerate a first set of serial data, the first set of serial datacomprising each bit of a first set of parallel data arranged one bitafter another in succession; buffer circuitry operable to generate asecond set of parallel data, the second set of parallel data comprisingmore than two instances, arranged in parallel, of each successive bit ofthe first set of serial data; and second serializer circuitry operableto generate a second set of serial data, the second set of serial datacomprising each bit of the second set of parallel data arranged one bitafter another in succession.
 10. The apparatus defined in claim 9,wherein width of the first set of parallel data is selectable.
 11. Theapparatus defined in claim 9, wherein width of the second set ofparallel data is selectable.
 12. The apparatus defined in claim 9,wherein the first serializer circuitry, the buffer circuitry, and thesecond serializer circuitry are coupled in series with one another, inorder, from the first serializer circuitry through the buffer circuitryto the second serializer circuitry.